1. Field of the Invention
The present invention relates generally to solid-state imaging devices, and more particularly to a semiconductor image sensor employing a charge transfer device such as a charge coupled device.
2. Description of the Related Art
Recently, as charge coupled devices (CCDs) have been much improved in the integration density of photosensitive cells, their applicability has been expanded. In particular, highly integrated CCD image sensors are increasingly used as the image sensing units for small-sized electronic home-use video cameras. Typically, the CCD device includes a planar matrix configuration of picture elements arranged on the substrate. The cell matrix essentially consists of an array of rows and columns of picture elements (referred to as "pixels" among those skilled in the art). Each pixel has a photodiode acting as a photosensing section for producing electrical charge carriers (generally called the "signal carriers") in response to an incident light and temporarily storing the carriers therein. The signal carriers read out from each column of pixels are then transferred to an output section of the CCD device through a corresponding vertical transfer CCD section associated therewith and a horizontal transfer CCD section coupled thereto.
According to expanding demands for further improvement in the integration density, when the allocated area for each pixel on the substrate surface decreases, it becomes more difficult to maintain good quality of images sensed by a presently available CCD image sensing device. The major reason for this is as follows. The CCD image sensor basically employs a PN-junction semiconductor photodiode as its photosensitive cell element. The photodiode is formed in a selected pixel area on the top surface of a silicon substrate. The substrate surface is covered by a transparent dielectric layer such as a silicon oxide film. A multi-stage transfer electrode, which defines a vertical charge-transfer section associated with a corresponding column of pixels, is buried in the dielectric layer. The dielectric layer is of uniform thickness on the substrate. A shielding electrode is disposed on the dielectric layer and has an opening that is formed therein to define a light-reception area of one pixel.
The pixel opening of the shielding electrode is arranged to partly overlap the PN-junction section of photodiode. In other words, the shielding electrode overlies the vertical transfer section, and its side end portion for defining the opening edge overlaps the peripheral area of the PN-junction photodiode section to provide an "overhung" portion. This overhung portion may function to cut off a leak component of incident light, which tends to straggle or "migrate" into the vertical charge-transfer section. This may be similar in function to eaves of an ordinary house. It will possibly happen for the incident light to be partially "straggled" to enter the vertical transfer section, rather than into the aimed PN-junction photodiode, due to what is called the "multi-reflection" in the transparent dielectric layer between the substrate surface and the shielding electrode. Such a light leakage to the vertical transfer section behaves badly to reduce the effective (net) amount of signal carriers, which leads to undesirable generation of a false image, such as "smear," on a sensed image. The aforementioned overhung portion of the shielding electrode must be required to eliminate such image-quality degradation.
The overhung arrangement, however, suffers from very serious conflicting problems as will be described below. As the integration density of CCD image sensors is being further improved, the occupation area of each pixel on the substrate will decrease. This will cause the overhung (or "eaves") portion of the shielding electrode to be narrower. This should be required because, if the overhung portion is simply kept constant while the cell size is decreased, the effective photosensing area of cell opening will obviously become smaller. The reduction of effective cell area will limit the amount of sensible light that is permitted to enter the photodiode. The resultant amount of signal carriers to be photoelectrically produced therein will also be reduced accordingly. This brings another problem: reduction of the signal-to-noise ratio of the image sensor. In view of the above, the elimination of leak light component and the accomplishment of higher integration density are technically in conflict with each other. Solving these conflicting problems has been long desired by the semiconductor manufacturers.